Serial changes in BMIPP uptake in relation to thallium uptake in the rat myocardium after ischaemia

Imaging of Heart Type Fatty Acid Binding Protein Under Acute Reperfusion Ischemia Using Radio-labeled Antibody in Rat Heart Model

Purpose: Heart-type fatty acid binding protein (H-FABP) is primary transporter of free fatty acid and plays an important role in myocardial metabolism, which is characterized by high specificity and rapid appearance under ischemic condition. The objective of this study was to clarify the usefulness of imaging study of targeting H-FABP appearance using radio-labeled antibody, and correlation with myocardial fatty acid metabolism and perfusion in acute reperfusion ischemia. Method: Wistar rats were allotted to sham-operated control group (sham; n=4), ischemia non-reperfused group (IG; n=5), and ischemia-reperfusion group (RG; n=5). Ligation of left coronary artery (LCA) was performed for IG and RG. 20 min of ischemia was followed by 60min of reperfusion for RG. ¹²⁵I labeled anti H-FABP antibody (anti H-FABP), BMIPP and ^99mTc-sestamibi (MIBI) was injected intravenously. Multi-tracer digital autoradiogram was performed using µ-imager^®. The ratio of radioactivity in LCA related (culprit) area to the inferior (remote) area (target uptake ratio=TUR) was generated. Results: In sham group, no visually detectable accumulation was observed for the anti H-FABP image, and TUR_MIBI and TUR_BMIPP were equivalent to 1. In IG, TUR_MIBI and TUR_BMIPP were remarkably low (0.12±0.01, 0.24±0.07). In RG, TUR_MIBI was significantly lower (0.20±0.03, p<0.05 vs. other groups). However, TUR_BMIPP was significantly higher (2.78±1.28, p<0.05) compared to the sham and IG, whereas anti H-FABP showed markedly higher ratio in the reperfused area compared to the sham and IG (3.43±0.73 vs. 0.31±0.13 and 1.09±0.07 for IG and sham; p<0.05, and <0.01, respectively). Conclusion: Anti H-FABP accumulated specifically in reperfused area under acute ischemia, and it accorded to the area where fatty acid metabolism was activated. This study has shown the future potential for clinical application in vivo imaging of acute coronary syndrome.

Discrepancy between myocardial perfusion and fatty acid metabolism following acute myocardial infarction for evaluating the dysfunctional viable myocardium

OBJECTIVE

Following acute myocardial infarction (AMI) the area of myocardial perfusion and metabolism mismatch is designated as dysfunctional viable myocardium. (123)I-beta-methyl iodophenyl pentadecanoic acid (BMIPP) is clinically very useful for evaluating myocardial fatty acid metabolism, and (99)mTc-Tetrofosmin (TF) is a widely used tracer for myocardial perfusion. This study was designed to evaluate the degree of discrepancy between BMIPP and TF at the subacute state of AMI.

METHODS

Fifty-two patients (aged 59 ± 10 years; mean 46 years) with AMI were enrolled, and all of them underwent percutaneous coronary intervention (PCI). Patients were classified according to ST-T change and PCI timing. (123)I-beta-methyl iodophenyl pentadecanoic acid and TF cardiac scintigraphy were performed on 7 ± 3.5 days of admission using a dual headed gamma camera. Perfusion and fatty acid metabolism defect were scored on a 17 segments model.

RESULTS

The mean BMIPP defect score on early and delayed images were 16.67 ± 10.19 and 16.25 ± 10.40, respectively. The mean TF defect score was 10 ± 7.69. Defect score of BMIPP was significantly higher than that of the TF (P < 0.0001; 95% CI 4.32-7.02), and there was a strong correlation between perfusion and metabolism defect score (r = 0.89, P < 0.00001). Forty-seven (90%) patients showed mismatched defect (BMIPP > TF), and 5 (10%) patients showed matched defect (BMIPP = TF). Mismatched defect score (MMDS) was significantly higher in patients with ST-segment elevation myocardial infarction (STEMI) than that of non-ST-segment elevation myocardial infarction (NSTEMI) (P < 0.041; 95% CI 0.11-5.19).

CONCLUSION

At the subacute state of AMI, most of the patients showed perfusion-metabolism mismatch, which represents the dysfunctional viable myocardium, and patients with STEMI showed higher mismatch.

Accelerated BMIPP uptake immediately after reperfused ischemia in the isolated rat heart model

OBJECTIVE

123I-beta-methyl iodophenyl pentadecanoic acid (BMIPP) can visualize myocardial fatty acid metabolism and has extensive potential for diagnosing cardiac diseases such as acute coronary syndrome in the clinical setting. Increased BMIPP uptake with decreased perfusion occasionally occurs under acute reperfusion ischemia and the kinetics of BMIPP remain unclear. The present study uses the isolated rat heart model to measure kinetic changes in BMIPP under acute reperfusion ischemia.

METHODS

Male Wistar rats were allotted to normal control (NG), mild (MG) and severe (SG) ischemia groups. The hearts were perfused according to the Langendorff method at a constant flow rate, and BMIPP wash-in and wash-out were studied. No-flow ischemia was applied for 15 and 30 min to the MG and SG groups, followed immediately by the wash-in and wash-out study. Whole heart radioactivity was determined using an external gamma detector throughout the experiment. Rates of myocardial uptake (K1, mL/min) and clearance (k2, min(-1)) were generated using a compartmental model analysis. The same procedures and protocols were performed using (99m)Tc-sestamibi (MIBI) as a perfusion study.

RESULTS

Perfusion pressure significantly increased and mean heart rate significantly decreased in the severe ischemia group (heart rate: 244 ± 76, 304 ± 105 and 94 ± 140 bpm; perfusion pressure: 67 ± 13, 101 ± 31 and 160 ± 84 mmHg for NG, MG and SG, respectively). MIBI-K1 significantly decreased, whereas BMIPP-K1 increased in the MG and SG groups (MIBI-K1: 3.45 ± 1.10, 1.95 ± 0.82, and 1.05 ± 0.13 mL/min; BMIPP-K (1): 3.06 ± 0.88, 3.91 ± 0.87, and 4.94 ± 1.51 mL/min for NG, MG and SG, respectively) with an inverse relationship to the severity of ischemia. MIBI-k2 increased markedly in severe ischemia (NG vs. MG: p < 0.05), whereas BMIPP-k2 did not change in the ischemic groups (MIBI-k2: 0.00072 ± 0.0011, 0.00038 ± 0.00076 and 0.043 ± 0.033; BMIPP-k2: 0.0056 ± 0.0028, 0.0029 ± 0.0010 and 0.0037 ± 0.0022 min(-1) for NG, MG and SG, respectively).

CONCLUSION

Myocardial BMIPP uptake increased immediately upon reperfusion after no-flow ischemia, and was inversely related to the severity of ischemia. The increased uptake was not due to reduced clearance, but to accelerated extraction.

In vivo myocardial infarct area at risk assessment in the rat using manganese enhanced magnetic resonance imaging (MEMRI) at 1.5T

The aim of this study was to measure the myocardial area at risk in rat, using MRI and manganese injection during a coronary occlusion/reperfusion model at 1.5T. A sequential protocol with occlusion and MnCl2 injection immediately followed by MRI was used with the assumption that MnCl2-induced contrast persistence is enough to accurately image the area at risk 90 min after occlusion. A total of 15 adult rats underwent a single 30-min episode of coronary occlusion followed by reperfusion. MnCl2 was injected (25 micromol/kg) at the beginning of the occlusion for 11 rats (group 1) and 6 h after reperfusion for four animals (group 2). A deficit of signal enhancement was observed in all rats. Hypoenhancement area in group 1 was correlated to the area at risk delineated by methylene blue (r=0.96, P<0.0001) whereas in group 2 it was correlated to the infarct area given by triphenyltetrazolium chloride (TTC) solution (r=0.98, P=0.003). The area at risk size was significantly correlated with left ventricle ejection fraction (LVEF), end-systolic volume and anterolateral wall thickening. This work demonstrates that hypoenhanced zone obtained after manganese injection during occlusion represents the area at risk and not only the infarct zone.

Long chain fatty acid uptake in vivo: comparison of [125I]-BMIPP and [3H]-bromopalmitate

Insulin resistance is characterized by increased metabolic uptake of fatty acids. Accordingly, techniques to examine in vivo shifts in fatty acid metabolism are of value in both clinical and experimental settings. Partially metabolizable long chain fatty acid (LCFA) tracers have been recently developed and employed for this purpose: [9,10-3H]-(R)-2-bromopalmitate ([3H]-BROMO) and [125I]-15-(rho-iodophenyl)-3-R,S-methylpentadecanoic acid ([125I]-BMIPP). These analogues are taken up like native fatty acids, but once inside the cell do not directly enter beta-oxidation. Rather, they become trapped in the slower processes of omega and alpha-oxidation. Study aims were to (1) simultaneously assess and compare [3H]-BROMO and [125I]-BMIPP and (2) determine if tracer breakdown is affected by elevated metabolic demands. Catheters were implanted in a carotid artery and jugular vein of Sprague-Dawley rats. Following 5 days recovery, fasted animals (5 h) underwent a rest (n = 8) or exercise (n = 8) (0.6 mi/h) protocol. An instantaneous bolus containing both [3H]-BROMO and [125I]-BMIPP was administered to determine LCFA uptake. No significant difference between [125I]-BMIPP and [3H]-BROMO uptake was found in cardiac or skeletal muscle during rest or exercise. In liver, rates of uptake were more than doubled with [3H]-BROMO compared to [125I]-BMIPP. Analysis of tracer conversion by TLC demonstrated no difference at rest. Exercise resulted in greater metabolism and excretion of tracers with approximately 37% and approximately 53% of [125I]-BMIPP and [3H]-BROMO present in conversion products at 40 min. In conclusion, [3H]-BROMO and [125I]-BMIPP are indistinguishable for the determination of tissue kinetics at rest in skeletal and cardiac muscle. Exercise preferentially exacerbates the breakdown of [3H]-BROMO, making [125I]-BMIPP the analogue of choice for prolonged (>30 min) experimental protocols with elevated metabolic demands.

Metabolic imaging using SPECT

INTRODUCTION

In normal condition, the heart obtains more than two-thirds of its energy from the oxidative metabolism of long chain fatty acids, although a wide variety of substrates such as glucose, lactate, ketone bodies and amino acids are also utilised. In ischaemic myocardium, on the other hand, oxidative metabolism of free fatty acid is suppressed and anaerobic glucose metabolism plays a major role in residual oxidative metabolism. Therefore, metabolic imaging can be an important technique for the assessment of various cardiac diseases and conditions.

MATERIALS AND METHODS

In SPECT, several iodinated fatty acid traces have been introduced and studied. Of these, (123)I-labelled 15-(p-iodophenyl)3-R, S-methylpentadecanoic acid (BMIPP) has been the most commonly used tracer in clinical studies, especially in some of the European countries and Japan.

RESULTS AND DISCUSSION

In this review article, several fatty acid tracers for SPECT are characterised, and the mechanism of uptake and clinical utility of BMIPP are discussed in detail.

Delayed recovery of fatty acid metabolism after transient myocardial ischemia: a potential imaging target for "ischemic memory"

The myocardium preferentially oxidizes free fatty acids for energy production. However, the dependency of this metabolic pathway on oxygen makes this process vulnerable to ischemia. The energy requirements of the myocardium are subsequently met by the oxidation of carbohydrates, particularly glucose. Recovery of fatty acid metabolism lags behind restoration of perfusion, resulting in the phenomenon of metabolic stunning. This decrease of fatty acid utilization following ischemia can be imaged with fatty acid radiotracers, particularly beta-Methyl-p-(123)I-iodophenyl pentadecanoic acid (BMIPP), which demonstrates markedly limited metabolism via beta-oxidation, resulting in prolonged retention in the cardiomyocyte. Thus, in patients presenting with chest pain and no prior myocardial infarction, abnormal BMIPP uptake at rest reflects metabolic alteration caused by the preceding ischemia, also termed ischemic memory.